The present invention relates to correcting B0 in echo-planar (EP) based magnetic resonance imaging (MRI), and more particularly to compensating for B0 drift and respiratory motion in magnetic resonance thermometry applications.
Magnetic resonance (MR) thermometry refers to tracking temperature changes in tissue of a patient using magnetic resonance image (MRI). An important application for MR thermometry is for monitoring temperature changes due to thermal therapy. Thermal therapy refers to the ablation of tissue of a patient using hot or cold temperatures. Thermal therapy can be advantageous to surgical, chemotherapy, and radiotherapy applications. Clinical examples of thermal therapy include Laser Induced Thermotherapy (LITT), High Intensity Focused Ultrasound (HIFU), Radio-frequency Ablation (RFA), and cryo-ablation.
B0 refers to the main magnetic field of an MR scanner. In thermal therapy, thermal dose induced temperature changes introduce shifts in the main magnetic field B0. Such B0 shifts cause changes to the MR phase images. Accordingly, a temperature map can be generated showing change in temperature in a patient's tissues due to thermal therapy by comparing the MR phase images acquired before and during the thermal therapy. However, B0 can be caused by multiple sources, such as the susceptibility differences between different tissue types, systematic B0 drift and respiratory motion, and temperature changes. As a result, the temperature map showing temperature changes due to thermal therapy is often inaccurate, since the measured temperature often carries reflects B0 shifts due to the anatomy, systematic drift and respiratory motion, in addition to the temperature change. That is, B0 shifts due to the anatomy, systematic drift and respiratory motion can cause artifacts in the temperature map. Accordingly, a method of compensating for B0 shifts due to the anatomy, systematic drift and respiratory motion is desirable.